206 Mr. F. Guthrie on Salt Solutions 



the insufficiency of present theories of magnetism to explain 

 the various peculiarities of the temporary magnetism of steel ; 

 and we have suggested that, in regard to the magnetic proper- 

 ties of its elements, that substance must be considered a hetero- 

 geneous mass. 



XXIV. On Salt Solutions and Attached Water. 

 By Frederick Guthrie*. 



II. 



Cryogens and Cryohydrates. 



BY Cryogen I mean an appliance for obtaining a temperature 

 below 0° C. In this paper it always signifies a freezing- 

 mixture. By Cryohydrate I mean the body resulting from the 

 union of water with another body, "and which hydrate can only 

 exist in the solid form below 0° C. As this communication is 

 in every respect continuous with the one brought before the 

 Physical Society on November 7th, 1874, and published in the 

 Philosophical Magazine for January 1875, the paragraphs are 

 numbered in sequence with those of the previous communication. 



Cryogens. 

 Precision of Temperature of Freezing-mixtures, 



§41. In § 39, when I was speaking of the possible uses of 

 cryohydrates for the maintenance of constant temperatures, 1 

 said : — " With the exception of the melting-points of a few 

 organic bodies such as benzol, and the boiling-points of a few 

 liquids such as liquid ammonia, sulphurous acid, and carbonic 

 acid, and the rather ill-defined temperatures to be got by various 

 freezing-mixtures, there are no means in the hands of physicists 

 for obtaining and maintaining with certainty and ease a fixed 

 temperature below 0° C." 



In regard to freezing-mixtures, I confess to have been here 

 very much misled by the confident but rather erroneous statements 

 of others, to which I attached faith trebly blind — blind, because no 

 recorded experiments really support them, blinder still because 

 a little thought in the right direction must have shown their 

 fallacy, and blindest of all because the one experiment of my 

 own in this direction (§§ 16 and 17) shows that the minimum 

 temperature of an ice-salt cryogen is reached whether we take 

 the ratios 3 of salt to 1 of ice or 1 of salt to 2 of ice, and so points 

 to the wideness of the margin of ratio which may obtain between 

 the weights of ice and the salt. It will further be shown in 

 * Communicated to the Physical Society, Jan. 18th, 1875. 



